1. Field of the Invention
The present invention relates to a digital signal processing system, and in particular to a receiver for processing carrierless AM/PM (CAP) signals.
2. Description of the Related Art
CAP transceivers are used to transmit and receive modulated signals over a transmission medium, such as an asymmetric digital subscriber line (ADSL) application. CAP is a bandwidth-efficient two-dimensional pass band line code in which the symbol data is organized in I and Q pairs. In this system, the I and Q data are filtered with orthogonal I and Q band pass filters having a common pass band. CAP is primarily intended for use in relatively simple communication links, where there is only one frequency division per channel. With CAP, all processing is done in the pass band of the filters, which eliminates the need for a carrier tracking loop (CTL), at the expense of tighter symbol timing constraints due to the frequencies of the pulses transmitted, which are higher than those of many other modulation schemes. CAP systems are described in detail in Werner, J. J., Tutorial on Carrierless AM/PM, Jun. 23, 1992 (UTP Development Forum, ANSI X3T9.5 TP/PMD Working Group).
Many end users do not have CAP transceivers capable of receiving such CAP signals. For example, many consumers have existing cable set-top transceiver devices which are not compatible with CAP. Because of the often prohibitive cost of installing new CAP receivers compatible with CAP signals, it is desirable to modify existing receivers to be able to receive CAP signals.